Ideally right after you finish building it. I have been using the line of Penofin products now for over 15 years....they make a product for just about any kind of deck you can imagine...from synthetics to tropical hardwoods.
Their product fall in the "penetrating oil" variety....which I feel is far better than any water based product.
Ideally you need to wait around six or seven weeks before you seal it this is to allow some of the moisture within the deck boards to dry out. If the deck has been getting wet during the waiting period you should extend that time before doing anything. With the older treated deck lumber you needed to wait close to a year, but new deck material, 7 months should be about right.
Penofin's stance is to protect the wood from the very start...This excerpt is from their web page.
Use Protech Cleaner to remove surface dirt and kill mold and mildew spores that may be living in your new wood. Follow the easy directions on the label for fast and effective cleaning.
It is our recommendation to apply a light coat of Penofin Oil Wood Finish to your wood surface while it is "seasoning". Why let the damage occur? Protect your investment from the beginning!
If you have allowed the wood surface to "season" or age before finishing, use Protech Brightener to remove any discoloration caused by tannins.
Some folks do wait a few weeks to treat Pressure treated wood...Personally I do not recommend using pressure treated wood for the "decking" or use surfaces....but do recommend it for the framing. I have never been happy with the Looks, splinters, checking etc of pressure treated wood.
Redwood and cedar being natural woods can be oiled as soon as the deck is complete...just like with Ipe, and the other hardwoods.
KMS, I would still be leery of putting anything on the wood until it drys out. While what they say sort of makes sense, the facts that I got from the manufactures of the wood say otherwise. Besides the wood will not develop mold if its drying out. If it keeps getting wet from rain, morning dew etc, then that may be a different story, But moisture will not allow any stains, or paints to properly grip or absorb into the fibers of the surface. So any coatings used when wood is surface damp, will not work properly.
I am sure you have picked up some of this lumber for projects and found one board to weigh almost twice of another in the same pile. The idea is to allow all of the wood to contain as close to possible the same amount of internal moisture. I would use if I had it a moisture meter before doing any type of paint or stain on decks anyway.
We use a lot of Mahogany in our neck of the woods, Ceder is way to soft for deck and Redwood, that is a 2nd mortgage on the house to purchase.
I am with you on treated lumber, risks all sorts of issues with splinters etc.
We were told to wait a couple of years to treat/stain out deck 26 years ago. So when my usband tore it out in 2006 and built a new one, I did not treat/stain it for two years. But by then, some of the wood planks had waffled. Woodbridge sounds more efficient.
The reason Melissa you were told to wait then is that the methods of treatments used in the wood to preserve it were much different then those used today. Also the products used to seal decks and treated lumber has also changed quite a bit as well. So what you did is not that much out of line for the times.
You also need to take your local climate into account...out here in the arid west lumber behaves a bit different than in the humid east. With pressure treated lumber here it only take a few days to a week of exposure to our 8-10% relative humidity ( at least in the summer) to dry the surface of the wood to point where it can be treated. This is about the same time frame i use after "cleaning" a deck to ensure the surface has dried sufficiently.
The core of some lumber may still contain higher levels of moisture but the outer 1/8" or so is dry enough for most treatments. A penetrating oil still "breathes" while a paint would fail due to hydrostatic pressure from within.
It depends on what treatment of sealer is being used.
Being closer to the redwood forests...our redwood prices are a bit better out here.
In the last year or so...our local Home Depot has been stocking the new version of "Prowood" that uses a brownish chemical version of ACQ instead of the greenish one....are you folks on the East coast seeing the same stuff?
You also have to take the various type of pressure treated wood into consideration...Nearly all of my project here are built using Prowoods ACQ dry products...
They also recommend immediate treatment